Aircraft are known that include hydraulic systems for operating landing gear of an aircraft that includes at least one hydraulic pressure source and at least one hydraulic return. The hydraulic system for operating the landing gear comprises actuators for moving undercarriages, which actuators are connected by a hydraulic circuit comprising a first hydraulic line that, when pressurized, causes the undercarriages to be lowered, and a second hydraulic line, that, when pressurized, causes the undercarriages to be raised. Those lines are pressurized selectively by means of a hydraulic distribution member comprising a general isolation valve for selectively isolating the hydraulic circuit from the pressure source of the aircraft, and at least one selector for putting one of the lines into communication with pressure and the other line in communication with return.
In the event of a failure, for example if the selector remains stuck in an intermediate position in which it closes both lines and keeps hydraulic fluid captive in the actuators, lowering of the landing gear can be prevented. It is therefore important to provide the hydraulic circuit with means for enabling at least the raising line to be put into communication with the return so that the landing gear is not prevented from being lowered, at least under gravity. In this respect, it is known to provide the circuit with a general decompression valve that is controlled directly by the pilot by means of a cable, or else by an electric motor that is actuated when the pilot presses a switch to go to emergency extension mode. The decompression valve connects both lines to the return, thereby avoiding any fluid being held captive in the actuators and preventing the landing gear from being lowered.
In certain recent aircraft, proposals have been made for an architecture as shown diagrammatically in FIG. 1 in which there is no longer a general decompression valve, but rather a plurality of decompression valves relocated close to the undercarriages.
The operating system shown in this figure is intended to operate the undercarriages of the aircraft together with the associated wheel-bay doors. The description below is based on an example of an aircraft having two main undercarriages and one auxiliary undercarriage (nosewheel).
To operate the undercarriages, the system comprises various actuators, including:                jacks 1 for moving the undercarriages, in which jacks the annular chamber (through which the piston rod passes) is connected to return in this example, while the full chamber (without the piston rod) is connected to a hydraulic line 5 marked “UP” in the figure, such that the jacks 1 cause the undercarriages to be raised when the “UP” line 5 is under pressure, and said jacks 1 do not oppose the undercarriages lowering under the effect of gravity when the “UP” line 5 is connected to return;        locking and unlocking cylinders 2 for locking/unlocking the struts of the undercarriages, each having an annular chamber connected to the “UP” line 5, while its full chamber is connected to a hydraulic line 6 marked “DOWN”, such that said locking/unlocking cylinders 2 act to unlock the struts while the undercarriages are being raised, with the “UP” line 5 under pressure and the “DOWN” line 6 connected to return, and so that said locking/unlocking cylinders 2 act to lock the struts when the “UP” line 5 is connected to return and the “DOWN” line 6 is under pressure; and        hooks 3 for co-operating with catch ferrules disposed on the undercarriages to hold them in the raised position, and actuated by hydraulic cylinders 4 in which the full chambers are connected to the “UP” line 5 and the annular chambers are connected to the “DOWN” line 6, such that said cylinders act to cause the hooks to close onto the catch ferrules of the undercarriages in order to hold the undercarriages in the raised position after they have been raised with the “UP” line 5 under pressure and the “DOWN” line 6 connected to return, and said cylinders also act to release the catch ferrules and allow the undercarriages to be lowered when the “UP” line 5 is connected to return and the “DOWN” line 6 is under pressure.        
Upstream from the actuators, a selective distribution member enables pressure to be admitted into the landing gear operating system, and enables pressure to be distributed selectively to the “UP” line 5 or to the “DOWN” line 6, with the other line being connected to return.
More precisely, the distribution member comprises:                an insulating valve 10 controlled by a driver computer 50 and serving to admit or not admit pressure to the landing gear control circuit;        a cutoff valve 11 for performing a function that is described in greater detail below and that, by default, is open; and        a three-position selector 12 controlled by the driver computer 50 having a neutral first position for connecting both the “DOWN” line 6 and the “UP” line 5 to return, a lowering position connecting the “DOWN” line 6 to pressure and the “UP” line 5 to return, and a raising position for connecting the “UP” line 5 to pressure and the “DOWN” line 6 to return.        
As mentioned in the figure, although not shown, the hydraulic circuit includes a similar selector for the doors of the landing-gear wheel-bays, and downstream from the door selector, door actuators including door-moving jacks and hooks.
Operation is as follows. Starting from the situation in which the aircraft is in flight and the landing gear is raised, the driver computer 50 begins by controlling the isolation valve 10 to open, and thus to admit pressure to a feed line 20 going from the isolation valve 10 to the selector 12. The selector 12 is in the neutral position so both the “DOWN” line 6 and the “UP” line 5 are connected to return. The driver computer 50 then puts the selector 12 into the lowering position so that pressure is admitted to the “DOWN” line 6, with the “UP” line 5 being kept connected to return. The doors open, the hooks 3 are controlled to release the undercarriages, and the undercarriages move downwards under gravity, to a deployed position in which the locking/unlocking cylinders 2 force the struts to lock, thereby stabilizing the undercarriages in their deployed positions.
When the aircraft has landed and subsequently taken off again, it suffices to place the selector 12 in the raising position so that the “UP” line 5 is pressurized while the “DOWN” line 6 is kept connected to return. The locking/unlocking cylinders 2 release the struts, thereby enabling the undercarriages to be raised under drive from the jacks 1. Once the raised position is reached, the hooks 3 hook onto the catch ferrules of the undercarriages so as to hold them in the raised position. The doors are then reclosed, and the selector 12 (and also the door selector) is returned to the neutral position.
It then suffices to cease applying a control signal to the isolation valve 10 so that it returns to its rest position in which the selector feed line 20 is connected to return.
Certain failure configurations, although statistically most unlikely, could prevent the landing gear from deploying. By way of example, the configuration is considered in which the selector 12 remains jammed in an intermediate position in which it closes off both the “DOWN” line 6 and the “UP” line 5. Under such circumstances, hydraulic fluid cannot be expelled from the full chamber of the jacks 1, so the undercarriages are blocked. To mitigate this drawback, the circuit includes three decompression valves 13 located close to the undercarriages. The decompression valves 13 are normally closed, but in an emergency they can be controlled to connect the “DOWN” line 6 and the “UP” line 5 to return, so as to eliminate any risk of hydraulic fluid being held captive in the chambers of the actuators 1, 2, 4.
For this purpose, the decompression valves 13, the cutoff valves 11, and the hooks 3 are provided with respective electromechanical actuator members each constituted in this example by two electric motors under the control of an emergency computer 51 that is activated by the pilot in the event of failure in the normal operation of the circuit, and that takes over from the driver computer.
On being activated, the emergency computer 51 actuates the cutoff valve 12 to prevent pressure being admitted into the circuit, and connects the admission line 20 to return. The emergency computer then controls the decompression valves 13 so as to connect both the “DOWN” line 6 and the “UP” line 5 to return. Finally, the emergency computer 51 controls the hooks so that they release the undercarriages, with the undercarriages lowering under the effect of gravity down to the deployed position in which the struts are locked mechanically by springs, as is well known.